1. Field of the Invention
The present invention relates to a laser cutting apparatus and a laser cutting method which are adapted to cut a material by concentrating laser light into the material to separate the material into a plurality of regions.
2. Description of the Related Art
Hitherto, there are various techniques of separating a surface of a material, which is to be cut, into a plurality of areas by concentrating laser light into the material to thereby cut the material. An example of the techniques is a known blade dicing method adapted to rotate a circular blade, which has a width ranging from tens μm to hundreds μm, at a high speed in a case where a semiconductor substrate, such as a silicon wafer, is precisely cut into semiconductor chips, so that the substrate is abraded by abrasives provided on the surface of the blade to cut the substrate. According to this method, coolant water is sprayed onto a cutting surface so as to reduce heat generation and abrasion associated with the cutting of the substrate. At that time, dusts, such as chips produced by cutting the substrate itself, fine particles of abrasives, and particles of adhesive of an adhesive tape used to fix the substrate and a processing table, are mixed into coolant water and are widely scattered. Especially, in a case where the substrate is a semiconductor substrate, many minute function elements are formed on a surface of the substrate. Thus, there is a fear that the dusts seriously affect the reliability of the function elements.
Also the cutting of a substrate can be performed in a dry environment without using coolant water. Thus, a processing method of cutting the substrate by concentrating laser light having a wavelength, at which the laser light is highly absorbed by the substrate, onto a surface of the substrate is used. However, this method has drawbacks in that parts of the surface of the substrate, which surround a cut part, are thermally melted, that especially, in the case where the substrate is a semiconductor substrate, logic circuits provided on the substrate are damaged, and that similarly to the blade dicing, problems due to dusts occur, because laser processing proceeds while melting the substrate from a laser light incidence side to a laser light exit side, so that dusts produced by resolidification of melted materials adhere to the surface of the substrate.
Japanese Patent Application Laid-Open Nos. 2002-192370 and 2002-205180 disclose examples of the processing method of cutting a substrate by concentrating highly absorbable laser light into a substrate. These methods employ an internal processing region formed by concentrating laser light having a specific wavelength, at which the laser light has high permeability for the substrate, into the substrate as a starting point for cutting the substrate. Thus, no melted regions are formed on the surface of the substrate. Consequently, these methods enable dust-reduced cutting.
However, a starting point for cutting is limited by the aforementioned method to an internal processing region inside a material to be cut. Thus, it is difficult to precisely control the direction and the position of a crack that reaches the surface of the material from the starting point.
In a case where the length in the direction of depth of the internal processing region formed in the material to be cut is too long and, thus, a crack reaches the surface of the material, the crack appearing in the surface of the material causes a spurt of dust. Additionally, in a case where a functional device is formed on the surface of the material, the functional device is damaged.
Especially, in a case where the material to be cut is a silicon wafer, the development of the crack is easily affected by a crystal orientation. Therefore, in a case where there is a misalignment between a predetermined cutting line and the crystal orientation due to an industrial error caused when a silicon substrate and devices are formed, according to the aforementioned laser processing method, a crack deviating from the predetermined cutting line in the process of developing in the surface may break down logic circuits provided in device portions.
This problem is described below with reference to FIG. 15. In a case where a modified layer 102 is formed by concentrating laser light of a specific wavelength into a part of a predetermined depth in a silicon substrate 101 made of a single-crystal silicon having a front surface that is a (100) plane, a crack 103a, whose starting point is positioned at an end portion 102a of the modified layer 102 provided in the vicinity of the front surface, may reach the front surface. In this case, because a high-index crystal orientation plane is formed on the end portion 102a of the layer 102 modified by laser processing, the actual crack 103a is not an ideal crack 103 and is inclined in a direction along a cleavage plane of the single-crystal silicon, which is a (110) plane or a (111) plane. FIG. 15 schematically shows a case where the front surface is a (100) plane and where the cleavage surface is a (111) plane. Consequently, the front surface of the silicon substrate 101 is cut at a position largely deviating from a predetermined cutting line C. In a case where the modified layer 102 is formed deep in the substrate 101, sometimes, the distance between the end portion 102a and the front surface of the substrate 101 is too long, so that the cutting/separation of the substrate 101 cannot be achieved.
Especially, in a device substrate of a liquid discharge head in which a discharge port, such as an inkjet nozzle, is formed, an opening structure adapted to supply liquid, such as ink, is provided under the discharge port. Thus, a problem arises in that a crack extends to such a structure, so that the substrate is broken down. This problem often arises in a case where the thickness of the substrate is large relative to a modified layer formed inside the substrate.